The invention relates to a computer tomography apparatus which includes a scanning unit which includes a radiation source and a detector unit which is connected thereto in order to detect a conical radiation beam, emitted by the radiation source, after its passage through an examination zone or an object present therein, a drive device for producing a relative motion in the form of a helix, consisting of a rotation about an axis of rotation and an advance in the direction parallel to the axis of rotation, between the scanning unit and the examination zone or the object, and a reconstruction unit for reconstructing the spatial distribution of the absorption within the examination zone from the measuring data acquired by the detector unit within a detector window defined by the helix.
A computer tomography apparatus of this kind (also referred to as CT apparatus hereinafter) is known from the PCT application SE 98/00029, dated Jan. 14, 1998. For the reconstruction of the absorption distribution the known computer tomography apparatus takes into account only the measuring data which is present within a detector window which is defined in the direction of the z axis by the projection of two successive turns of the helix (in this context and hereinafter a detector window is to be understood to mean the part of the measuring surface of the detector unit which acquires exclusively the data required for the reconstruction). It can be demonstrated that when the detector window is configured in this manner, the radiation source projects each voxel in the examination zone, upon its entry into and its departure from the radiation beam, onto the detector window from exactly 180.degree. offset positions (relative to the respective voxel). The measuring data thus acquired enables exact reconstruction of the absorption distribution in the examination zone, even when the object present therein is longer than the part of the examination zone which is covered by the conical radiation beam.